As a motor having high efficiency and a wide variable speed range, a permanent magnetic synchronous motor (PMSM), in particular, an interior permanent magnetic synchronous motor (IPMSM) in which a permanent magnet is embedded in a rotor has found extensive applications such as a compressor driving motor of an air conditioner for vehicle and a drive motor for electric automobile. Demand for the motor is expected.
A motor control device that controls driving of the motor of this type is composed of a motor, an inverter, a direct-current power supply, and a controller incorporating a microcomputer.
In the operation of the motor, in general, the controller detects an electric current flowing through a coil wound around a stator (armature) of the motor and causes the electric current to follow a target current phase through current feedback control. In the current feedback control, the controller decomposes the target current phase into a d-axis current Id, which is a d-axis component parallel to a magnetic field, and a q-axis current Iq, which is a q-axis component orthogonal to the magnetic field, and sets, as the target current phase, a current vector composed from the d-axis current Id and the q-axis current Iq on a d-q-axis coordinate, and controls the current vector. Consequently, it is possible to highly efficiently operate the motor at optimum torque.
Specifically, there is known a current vector control method called maximum torque/current control, which includes setting, as a target current phase, a current phase in which generated torque of a motor is maximized with respect to a detected electric current. For example, Patent Literature 1 discloses the following general expression representing a relation between d and q-axis currents in performing the maximum torque/current control:
                              i          d                =                                            Φ              a                                      2              ⁢                              (                                                      L                    q                                    -                                      L                    d                                                  )                                              -                                                                      Φ                  a                  2                                                  4                  ⁢                                                            (                                                                        L                          q                                                -                                                  L                          d                                                                    )                                        2                                                              +                              i                q                2                                                                        [                  Expression          ⁢                                          ⁢          1                ]            
In the expression, id: d-axis current, iq: q-axis current, Φa: permanent magnet magnetic flux, Lq: q-axis inductance, and Ld: d-axis inductance. In this case, the target current phase is set to maximize the generated torque of the motor with respect to the detected electric current without depending on the number of revolutions (angular velocity) ω of a rotor, i.e., the motor.
On the other hand, in the motor, it is a common practice to use so-called sensorless control, which includes detecting an induced voltage of the motor from, for example, information concerning an electric current and a voltage detected by a controller and effectively detecting a rotor position to control the motor without using a physical sensor. Actual d and q axes are not directly known during the sensorless control. Therefore, the controller sets imaginary axes respectively for the original d and q axes and executes the current vector control on the imaginary axes.
However, the imaginary axes are only axes assumed in the controller. Therefore, an angle error of Δθr is present between the actual d and q axes. In order to efficiently stably operate the motor, it is necessary to set a value of Δθr as small as possible.
For example, Patent Literature 2 discloses the following axial position estimation expression:
                              Δ          ⁢                                          ⁢          θ                ≈                              tan                          -              1                                ⁡                      [                                                            V                                      d                    ⁢                                                                                  ⁢                    c                                                  -                                  r                  ·                                      I                                          d                      ⁢                                                                                          ⁢                      c                                                                      +                                                      ω                    1                                    ⁢                                                            L                      q                                        ·                                          I                      qc                                                                                                                    V                                      q                    ⁢                                                                                  ⁢                    c                                                  -                                  r                  ·                                      I                                          q                      ⁢                                                                                          ⁢                      c                                                                      -                                                      ω                    1                                    ⁢                                                            L                      q                                        ·                                          I                                              d                        ⁢                                                                                                  ⁢                        c                                                                                                                  ]                                              [                  Expression          ⁢                                          ⁢          2                ]            
In the expression, Δθ: axial position estimation error (rotor position error, current phase error), Vdc: d-axis component of an applied voltage, Vqc: q-axis component of the applied voltage, Idc: d-axis current, Iqc: q-axis current, Lq: q-axis inductance, Ld: d-axis inductance, r: winding resistance of a coil, and ω1: frequency of the applied voltage. All of Vdc, Vqc, Idc, and Iqc described above are assumed values in the controller premised on the imaginary axes, all of Lq, Ld, and r described above are machine constants of the motor, and ω1 is a measured value. During the sensorless control, the controller performs control in order to converge Δθ described above to zero.